1. Field of the Invention
The present invention is in the field of manufacturing film suitable for use as laminating film and involves coextrusion of a plurality of films of different chemical composition, biaxially orienting the same, subjecting the biaxially oriented tube to controlled sizing conditions, and then forming a pair of flat films from the tube.
2. Description of the Prior Art
Various types of heat-activated lamination systems have been used commercially for many years. Such systems are usually used for encapsulating and sealing of documents between two plastic films to protect documents and to enhance the visual appearance. Such post-lamination is usually performed in offices, schools, or graphic art shops. In the case of laminating identification cards, licenses, and the like, there are often stringent specifications and consistent high quality results are required.
The laminating films used in this type of lamination procedure usually consist of one layer of unoriented polyethylene or a copolymer thereof, and another layer of oriented and heat-set polyethylene terephthalate (PET). Typically, these materials are prepared by an extrusion coating of polyethylene or its copolymers onto an oriented PET film web.
The extrusion processes commonly used to produce packaging films, although producing usable laminating films, provide several inherent drawbacks. For one, hazy streaks and bubbles occasionally will occur as a result of contact with the hot shoes. These streaks and bubbles are caused by air bubbles which are trapped at the interface between the two layers during the coating operation. Such bubbles are usually microscopic in size but they expand into larger bubbles and cause visual defects such as streaks when subjected to certain heating conditions in the lamination step. This phenomenon only occurs from time to time. It has been known that it is influenced by process conditions, but it has not been known how to predictably cure it.
Heated roll laminators also have their own disadvantages, particularly the creation of wrinkles and the presence of uneven heating. For post-lamination applications, it is desirable that the film should shrink minutely in the transverse direction when heated so as to smooth out the film which, in turn, insures uniform heating. Laminating film produced by existing extrusion coating operations occasionally will exhibit positive dimensional change in the transverse direction of the film when heated. Although the amount of change is very small, it is very undesirable since it causes the film to wrinkle and to be heated unevenly. The reasons for this transverse growth are believed to be twofold. In the extrusion coating operation, the PET is usually heated to drive off the solvent or water from the primers. At the same time, the fi1m is necessarily under tension in the machine direction (MD) to avoid wrinkles and the like. This combination of conditions tends to cause tensile stress in the PET film in the machine direction, and, due to the poisson's ratio of the material, a certain amount of compression stress is also effected in the transverse direction of the film. The stresses are frozen-in when film is cooled under tension. When the film is re-heated in the post-lamination step, the stresses are relieved to cause MD shrinkage and transverse growth.
Moreover, the PET film when produced by conventional flat cast procedures tends to have low transverse shrinkage which contributes to the aforementioned transverse growth problem.
Another drawback existing in conventional laminating film is inadequate interfacial adhesion for certain applications. In the manufacture of identification cards, outdoor signs, and the like, the resulting laminates must possess excellent interfacial adhesion in laminating films but must also be resistant to environmental influences such as high humidity, rain, and the like. This requirement is not met by existing laminating film technology. Although many water-resistant primers are available, they all contain residual low molecular weight fractions which, upon heating in the lamination step, tend to migrate through the polyethylene layer to the surface, causing a weakening effect on the adhesion of the laminate to the underlying document.
Another defect results from the fact that in post-laminating, the film is always contact heated either by means of heated rollers or by heated shoes. Uniform heating therefore depends on uniform contact. The presence of gauge bands which are inherent in flat die-casting and coating processes cause uneven contact, and thus cause uneven heating. Although good quality control on existing processes can reduce this problems, it cannot be eliminated completely.
There are numerous examples of multi-ply laminating films in the patented prior art, of which the following are believed to be typical examples.
Bornstein et al U.S. Pat. No. 4,064,296 describes a heat shrinkable, multi-layer film including a layer of a hydrolyzed ethylene-vinyl acetate copolymer formed by co-extruding the hydrolyzed ethylene-vinyl acetate copolymer layer between two other polymeric layers which themselves may be ethylene-vinyl acetate copolymers. The resulting laminated structure is thereafter irradiated and oriented to produce a film which is heat shrinkable and is said to have very low oxygen permeability.
Schirmer U.S. Pat. No. 4,095,012 describes a process for producing oriented films and laminates from nylon 66 and blends thereof wherein a nylon layer is co-extruded between layers of polymeric materials such as polyolefins and their copolymers to form a laminate. After quenching of the laminate, the laminated structure is oriented biaxially.
In U.S. Pat. No. 4,151,328 to Kight there is described a packaging material film including a self-adhering layer composed of a terpolymer of ethylene, a vinyl ester, and an alkenoic acid. This layer is applied onto a plasticized saran.
Mueller et al in U.S. Pat. No. 4,188,443 describes a multi-layer polyester/polyolefin shrink film consisting preferably of five layers in which the middle layer is a polyester or copolyester, the two inner layers adjacent the middle layer are both ethylene-vinyl acetate copolymers, and the skins or outer layers are ethylene-propylene copolymers. This patent describes biaxial stretching of the two by means of internal air pressure to form a bubble.
U.S. Pat. No. 4,197,326 to Wakamatsu et al describes a packaging tube composed of an oriented laminated film formed by stretching a composite film having an inner layer composed of an oxygen barrier thermoplastic resin confined between outer layers of a thermoplastic resin having a smaller water absorption capability than the inner layer. The oxygen barrier layer may be materials such as polyamides, PET, hydrolyzed ethylene-vinyl acetate copolymers (EVA) and polyvinyl alcohol whereas the thermoplastic resin outer films are composed of materials such as polyethylene, EVA, or polybutene.
Andrews et al U.S. Pat. No. 4,198,256 describes a heat-sealable oriented plastic film consisting of an oriented polypropylene film covered with a heat-sealable layer consisting of a predominantly linear random copolymer of ethylene with an additional alpha-olefin having at least three carbon atoms per molecule.
In U.S. Pat. No. 4,198,458 to Mitsuishi et al there is described a stretch-oriented laminate polyester film consisting of two or three layers of polyester films of different intrinsic viscosities. The preferred embodiment of the invention consists of a stretch-oriented laminated film and a magnetic layer coated on an external surface thereof to provide a magnetic recording medium.
U.S. Pat. No. 4,274,900 to Mueller et al describes a multi-layer polyester/polyolefin shrink film preferably having five layers in which the middle layer is a polyester or copolyester, the two inner adjacent layers are EVA copolymers, and the skin or outer layers comprise ethylene-propylene copolymers.